It is well known to operate a blast furnace with a blast of ambient air heated by one of a set of regenerative heaters, typically three hot-blast stoves (often called “Cowpers”). Each hot-blast stove is cyclically operating by switching between a heating phase (“on gas” or “off-blast” phase) and a blowing phase (“on-blast” phase). To this effect, a hot-blast stove has internal heat storage elements, typically checker bricks, and an associated burner for producing hot flue gas to heat the checker bricks. The burner may be internal or external. To permit hot gas to pass during the heating phases, the hot-blast stove has a heating gas inlet and a flue gas outlet. They permit heating gas to flow from the burner through the stove and its heat storage elements (checker bricks) and, through the flue gas outlet, to a flue gas stack or chimney. With an internal burner, the heating gas is produced by combustion inside the stove. For the heating of high-pressure blast air, a conventional hot-blast stove further has a cold blast inlet connected to a cold blast main and a hot blast outlet connected to a hot blast main of the blast furnace. During the blowing phases, air is blown from the cold blast inlet, through the regenerative heater where it is heated up by the heat storage elements and then fed to the blast furnace via the hot blast outlet. Regenerative heaters are used to heat the blast to a temperature in the range of 1100° C. to about 1250° C.
In recent years, the re-use of top gas in the blast received increasing attention, since it enables notable reductions of CO2 emissions. Corresponding installations recover blast furnace top gas and subject it, usually after conventional top gas cleaning, to a recycling process before injecting it back into the blast furnace. The recycling process includes CO2 separation for withdrawing CO2 from the process. To this effect, a gas separation unit separates top gas into tail gas that is rich in CO2 (carbon dioxide) and into high calorific value process gas, rich in CO (carbon monoxide). As suitable gas separation unit it has been suggested to use a pressure swing adsorption (PSA) or a vacuum pressure swing adsorption (VPSA) unit or, alternatively, a CO2 scrubber unit. The tail gas may be fed through a cryogenic unit to separate out pure CO2 or subjected to any other further processing, ideally resulting in CO2 capture and storage. The other gas stream, however, i.e. CO-enriched process gas, is fed back into the blast furnace as reducing gas, whereby altogether lower CO2 production is achieved.
The required heating of CO rich process gas may be carried out in regenerative heaters. However, the replacement of an ambient air as cold blast with CO rich process gas, i.e. a reducing gas, has considerable implications. In particular, special measures and precautions are necessary concerning the changeover sequences between the heating and blowing cycles and vice-versa.
Among others, gas fed to the regenerative heater during the heating phase is generally oxidizing and therefore liable to react explosively with high calorific process gas supplied during the blowing cycle. In order to avoid that any dangerous quantity of oxidizing gas is present in the regenerative heater during the changeover to the blowing phase, PAUL WURTH proposed, in PCT application WO2010/133476, a method of operating the burners in a manner that ensures that any oxygen is consumed in the regenerative heater. For the transition from the blowing phase to the heating phase, WO2010/133476 proposes to push out the residual CO containing process gas out of the regenerative heater with the flue gas of the burner.
However, WO2010/133476 is silent about specific measures related to the changeover from the blowing phase to the heating phase. In view of the fact that pressure in the regenerative heater during the blowing phase (on-blast pressure) is typically higher than pressure during the heating phase (on-gas pressure), it may be necessary to take specific measures before the burner(s) can be ignited.